To enhance the accuracy of sound reproduction, it is desirable to reproduce reflected sound or echoes. This component, often referred to as ambiance, is generated by sound reflection off the walls and other surfaces of a concert hall, and thus its source is generally behind or to the side of the listener. Quadrophonic sound recordings can reproduce ambiance by recording on their rear channels sound that emanates from the rear. Stereophonic recordings, on the other hand, lack these rear recorded channels. Various systems have been employed to create ambiance from the two stereo channels. One technique is to play a delayed version of the stereo channels from rear speakers. This works reasonably well for orchestral music in which variously located instruments play simultaneously. But the systems give an unnatural quality to the sound of a solo instrument or vocalist, including a radio announcer, making the sound appear to emanate from a cave.
Another approach to simulating live sound from two stereo channels is the so-called "surround sound" approach in which, e.g., pure left-channel sound appears to come from the left rear or side of the listener while monaural sound (i.e., sound common to both left and right channels) appears to come from between the front two speakers. Iida U.S. Pat. No. 3,725,586 discloses such a system. Each left and right channel is applied to a phase-shifting circuit and a low-pass filter. Forward speakers are driven by the outputs of the phase-shifting circuits. Rear speakers are driven by the sum of a filtered signal and a phase-shifted signal from opposite channels (e.g., left filtered plus right phase-shifted giving right rear). The relative phase shift between the summed signals is 180.degree. at about 650 to 700 Hz and approaches 270.degree. at higher frequencies. The monaural signal on the two channels is thereby attenuated in the rear speakers, most greatly at 650 to 700 Hz and to a lesser degree at higher frequencies. The filters are used to preserve left and right side localization of sound.